1. Field of the Invention
This invention relates to means for preventing fluid leakage from a shaft opening in a compressor casing or hydrogen cooled generator casing and more particularly to a sealing mechanism which utilizes a static seal piston during non-rotation of the shaft.
2. Description of the Prior Art
In axial compressors and hydrogen cooled generators the rotor shaft ends must be brought out of a fluid tight enclosure or casing, necessitating the use of some means to prevent escape of compressed fluid along the shaft to the atmosphere. During shaft rotation, a ring seal is commonly used. When shaft rotation ceases, it may still be desirable to prevent leakage if the compressed fluid within the casing is expensive and cannot be drawn off and trapped in another enclosure. For axial compressors used in refrigeration systems and for hydrogen cooled generators it is inconvenient and undesirable to draw off and trap the compressed fluid in another enclosure during non-rotation of the shaft. Ring seals and rubbing seals have often been used to trap the compressed fluids within the casings during shaft non-rotation as well as during shaft rotation, but these methods were judged not to be entirely satisfactory for all modes of operation.
The ring seal utilizes a set of annular plates which surround the shaft and are supplied with oil under pressure. The oil is distributed around the shaft by the annular plates with some oil being forced both ways along the shaft through a small clearance between the plates and shaft, thus preventing escape of compressed fluid from the casing. The ring seal containment method functions satisfactorily if the oil supply to the ring seal is maintained at a higher pressure than the pressure of the compressed fluid held within the casing.
The rubbing seal utilizes a sealing material which is held in contact with the shaft or collar attached thereto. The interface between the sealing material and shaft requires lubrication to prevent excessive heat generation, material failure, and rapid depletion of the sealing material. Even with lubrication the rubbing seal requires frequent and periodic replacement and often does not have sealing capability comparable to that of the ring seal.
In closed loop refrigeration systems when the compressor speed is reduced below the normal operating speed for either complete stopping or turn gear operation, the refrigerant pressure within the compressor can increase because of the phase change from liquid to gas of a large portion of the refrigerant trapped within the refrigeration loop. When natural gas is used as a refrigerant and the compressor speed is decreased below the normal operating speed, the compressor suction pressure can increase from 68 psig to 480 psig. Since the ring seal method requires the seal fluid pump to provide a higher discharge pressure than that of the natural gas, the seal fluid pump would consume a great deal of power when the compressor was not in the normal mode of operation. Another factor which makes the ring seal an unattractive sealing method during compressor shutdown is the enormous volume of sealing fluid required for large, axial compressor shafts and large generator shafts. The large shaft size and accompanying surface interface speeds also eliminate the rubbing seal from consideration because of its high wear rate and relatively large degree of leakage along the shaft.
A similar static seal piston is the topic of a copending application filed Nov. 29, 1976, having Serial No. 736,803, having the same assignee as the present application and having the following inventors: A. A. Raimondi, H. N. Kaufman, and N. J. Wood. A feature of the similar static seal piston is that if the ring seal or other dynamic sealing device fails to prevent leakage during shaft rotation, the static seal piston would automatically slide to the closed, sealing position severely damaging the static seal piston, sleeve, shaft shoulder, or deformable element. Rather than overhauling the sealing mechanism after such a ring seal failure, it may be preferable to prevent automatic closure at the expense of losing some of the sealed fluid in leakage to the atmosphere while the shaft is brought to rest.